U.S. patent number 8,936,684 [Application Number 13/308,344] was granted by the patent office on 2015-01-20 for vitreous silica crucible.
This patent grant is currently assigned to Japan Super Quartz Corporation. The grantee listed for this patent is Hiroshi Kishi, Ken Kitahara, Toshiaki Sudo. Invention is credited to Hiroshi Kishi, Ken Kitahara, Toshiaki Sudo.
United States Patent |
8,936,684 |
Sudo , et al. |
January 20, 2015 |
Vitreous silica crucible
Abstract
The present invention provides a vitreous silica crucible which
can suppress buckling and sidewall lowering of the crucible and the
generation of cracks. According to the present invention, a
vitreous silica crucible is provided for pulling a silicon single
crystal having a wall, the wall including a non-doped inner surface
layer made of natural vitreous silica or synthetic vitreous silica,
a mineralizing element-maldistributed vitreous silica layer
containing dispersed island regions each containing a mineralizing
element, and wherein the vitreous silica of the island regions and
the vitreous silica of a surrounding region of the island regions
is a combination of mineralizing element-doped natural vitreous
silica and non-doped synthetic vitreous silica, or a combination of
mineralizing element-doped synthetic vitreous silica and non-doped
natural vitreous silica, and the inner surface layer is made of
vitreous silica of a different kind from that of the island
region.
Inventors: |
Sudo; Toshiaki (Akita,
JP), Kishi; Hiroshi (Akita, JP), Kitahara;
Ken (Akita, JP) |
Applicant: |
Name |
City |
State |
Country |
Type |
Sudo; Toshiaki
Kishi; Hiroshi
Kitahara; Ken |
Akita
Akita
Akita |
N/A
N/A
N/A |
JP
JP
JP |
|
|
Assignee: |
Japan Super Quartz Corporation
(Akita-shi, JP)
|
Family
ID: |
45093498 |
Appl.
No.: |
13/308,344 |
Filed: |
November 30, 2011 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20120137964 A1 |
Jun 7, 2012 |
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Foreign Application Priority Data
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Dec 3, 2010 [JP] |
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2010-270925 |
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Current U.S.
Class: |
117/208; 117/200;
117/13; 117/11; 117/206 |
Current CPC
Class: |
C30B
15/10 (20130101); C03B 19/095 (20130101); Y10T
117/10 (20150115); Y10T 117/1032 (20150115); Y10T
117/1024 (20150115) |
Current International
Class: |
C30B
15/10 (20060101) |
Field of
Search: |
;117/11,13,200,206,208,928,931-932 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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0 463 543 |
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Jan 1992 |
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EP |
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0463543 |
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Jan 1992 |
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EP |
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2000-247778 |
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Sep 2000 |
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JP |
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2006-169084 |
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Jun 2006 |
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JP |
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2008-214189 |
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Sep 2008 |
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JP |
|
Other References
Extended European Search Report mailed Mar. 26, 2012, issued in
corresponding Application No. EP 11191170.7, 6 pages. cited by
applicant.
|
Primary Examiner: Bratland, Jr.; Kenneth A
Attorney, Agent or Firm: Christensen O'Connor Johnson
Kindness
Claims
What is claimed is:
1. A vitreous silica crucible for pulling a silicon single crystal
having a wall, the wall comprising, from an inner surface side
toward an outer surface side of the crucible, a non-doped inner
surface layer made of natural vitreous silica or synthetic vitreous
silica, a mineralizing element-maldistributed vitreous silica layer
containing dispersed island regions, each containing a mineralizing
element, and wherein the vitreous silica of the island regions and
the vitreous silica of a surrounding region of the island regions
is a combination of mineralizing element-doped natural vitreous
silica and non-doped synthetic vitreous silica, or a combination of
mineralizing element-doped synthetic vitreous silica and non-doped
natural vitreous silica, and the inner surface layer is made of
vitreous silica of a different kind from that of the island
regions.
2. The vitreous silica crucible of claim 1, wherein the average
diameter of the island regions is 50 .mu.m to 1 mm.
3. The vitreous silica crucible of claim 1, wherein the
concentration maximum value of a mineralizing element in the island
regions is 20 to 600 ppm.
4. The vitreous silica crucible of claim 1, wherein the
mineralizing element in the mineralizing element-maldistributed
vitreous silica layer is Al.
5. The vitreous silica crucible of claim 1, wherein the inner
surface layer is synthetic vitreous silica, and the vitreous silica
of the island regions and the vitreous silica of the surrounding
region is the combination of mineralizing element-doped natural
vitreous silica and non-doped synthetic vitreous silica.
6. The vitreous silica crucible of claim 1, wherein the inner
surface layer is natural vitreous silica, and the vitreous silica
of the island regions and the vitreous silica of the surrounding
region is the combination of mineralizing element-doped synthetic
vitreous silica and non-doped natural vitreous silica.
7. The vitreous silica crucible of claim 5, further comprising a
mineralizing element uniformly-distributed vitreous silica layer on
an outer side of the mineralizing element-maldistributed vitreous
silica layer, the mineralizing element uniformly-distributed
vitreous silica layer containing a substantially uniformly
dispersed mineralizing element, wherein the mineralizing element
uniformly-distributed vitreous silica layer is made of vitreous
silica of a different kind from the vitreous silica of the
surrounding region.
8. The vitreous silica crucible of claim 7, wherein the
concentration of the mineralizing element in the mineralizing
element uniformly-distributed vitreous silica layer is 20 to 600
ppm.
9. The vitreous silica crucible of claim 7, wherein the
concentration of the mineralizing element in the mineralizing
element uniformly-distributed vitreous silica layer is larger than
the concentration maximum value of the mineralizing element in the
island regions.
10. The vitreous silica crucible of claim 7, wherein the
mineralizing element in the mineralizing element
uniformly-distributed vitreous silica layer is Al.
11. The vitreous silica crucible of claim 1, wherein the
mineralizing element-maldistributed vitreous silica layer is formed
by fusing mixed silica powder obtained by mixing mineralizing
element-doped natural silica powder and non-doped synthetic silica
powder, or mineralizing element-doped synthetic silica powder and
non-doped natural silica powder in a ratio of 1:1 to 1:100.
Description
CROSS-REFERENCE TO RELATED APPLICATION
This application is related to Japanese Patent Application No.
2010-270925 filed on Dec. 3, 2010, whose priority is claimed and
the disclosure of which is incorporated by reference in its
entirety.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a vitreous silica crucible.
2. Description of the Related Art
In general, silicon single crystal is manufactured by melting
high-purity polycrystalline silicon in a vitreous silica crucible
to obtain a silicon melt, dipping an end of a seed crystal to the
silicon melt, and pulling the seed crystal while rotating it.
The melting point of silicon is 1410 degrees C., and thus the
temperature of the silicon melt is kept at a temperature equal to
or higher than 1410 degrees C. At such temperature, a vitreous
silica crucible reacts with the silicon melt, and the thickness of
the crucible wall gradually decreases. When the thickness of the
crucible wall decreases, the strength of the crucible is lowered.
This leads to problems such as buckling and sidewall lowering of
the crucible.
In order to solve such problems, there is known a technique to
provide a layer, on the outside of the crucible, to promote
crystallization. When such a layer is provided, the outer layer of
the crucible is crystallized and the crucible strength is improved
(e.g., JP-A-2000-247778). When such a layer is provided, the outer
layer of the crucible is crystallized when the crucible is heated
for a long time. Crystalline silica has higher strength per unit
thickness than vitreous silica. Therefore, the crystallization
enhances the strength per unit thickness, and suppresses buckling
or sidewall lowering of the crucible.
SUMMARY OF THE INVENTION
Conventionally, a vitreous silica crucible is used to pull a single
silicon ingot, and after the single pulling, the vitreous silica
crucible is discarded without being reused (such pulling is called
"single pulling"). However, nowadays, for the purpose of cost
reduction of a silicon ingot, a vitreous silica crucible is used
for multi-pulling, where after a first silicon ingot is pulled
using a vitreous silica crucible, the vitreous silica crucible is
re-used for pulling a second silicon ingot by re-charging and
melting polycrystalline silicon before the crucible is cooled down.
Thus, "multi-pulling" means pulling multiple silicon ingots using a
single vitreous silica crucible.
According to the technique of JP-A-2000-247778, an Al-added silica
layer is formed as the outer layer, for example, in a thickness of
4 mm, and when a crucible has such a thickness, the buckling and
sidewall lowering of the crucible can be effectively suppressed in
single pulling. However, in multi-pulling, the crucible is exposed
to a high temperature environment for a longer time, and thus the
reduction of the wall thickness and softening of the crucible is
more eminent, and thus the buckling and sidewall lowering of the
crucible cannot be sufficiently suppressed according to the
technique of JP-A-2000-247778, and thus the crucible strength needs
to be enhanced.
When the Al-added silica layer of the outer layer is thickened in
the technique of JP-A-2000-247778, a crystallized layer formed in
the outer layer is also thickened, and thus the crucible strength
is expected to increase. However, when the crystallized layer is
thickened, cracks are more likely to be formed in the crucible.
When the cracks are formed in the crucible, the high temperature
silicon melt inside the crucible leaks from the crucible, and all
surrounding devices are damaged. Therefore, it is difficult to
improve the crucible strength by thickening the crystallized layer
in the outer layer.
The present invention has been made in view of these circumstances,
and provides a vitreous silica crucible which can suppress buckling
and sidewall lowering of the crucible and generation of cracks.
According to the present invention, provided is a vitreous silica
crucible for pulling a silicon single crystal having a wall, the
wall comprising, from an inner surface side toward an outer surface
side of the crucible, a non-doped inner surface layer made of
natural vitreous silica or synthetic vitreous silica, a
mineralizing element-maldistributed vitreous silica layer
containing dispersed island regions each containing a mineralizing
element (hereinafter referred to as "mineralizing
element-maldistributed layer"), and wherein the vitreous silica of
the island regions and the vitreous silica of a surrounding region
of the island regions is a combination of mineralizing
element-doped natural vitreous silica and non-doped synthetic
vitreous silica, or a combination of mineralizing element-doped
synthetic vitreous silica and non-doped natural vitreous silica,
and the inner surface layer is made of vitreous silica of a
different kind from that of the island region.
The crucible of the present invention includes, on the outer side
of the non-doped inner surface layer, a mineralizing
element-maldistributed layer containing dispersed island regions
each containing a mineralizing element.
The mineralizing element has a function to promote crystallization
of a vitreous silica layer of a crucible when the crucible is
heated during pulling of a silicon ingot. When the mineralizing
element uniformly exists in the vitreous silica layer, the entire
vitreous silica layer is crystallized. Crystalline silica is more
difficult to be deformed than vitreous silica, and thus when the
entire vitreous silica layer is crystallized, cracks are easier to
be formed in the crucible.
In the present invention, the mineralizing element-maldistributed
layer contains dispersed island regions each containing a
mineralizing element, and thus this layer contains a portion whose
mineralizing element concentration is high and a portion
substantially not containing a mineralizing element. In the portion
substantially not containing a mineralizing element,
crystallization of vitreous silica is very slow, and thus
crystallization started in the portion whose mineralizing element
concentration is high is difficult to spread to the portion
substantially not containing a mineralizing element. As a result,
island-like crystals are formed in the vitreous silica layer. In
such configuration, when stress is applied to the vitreous silica
layer, vitreous silica portions existing between the adjacent
island-like crystals are deformed to suppress generation of cracks
in the crucible.
The vitreous silica of the island regions and the vitreous silica
of the surrounding region is a combination of mineralizing
element-doped natural vitreous silica and non-doped synthetic
vitreous silica. When both of the vitreous silica of the island
regions and the vitreous silica of the surrounding region are
natural vitreous silica or both are synthetic vitreous silica, the
crystallization started at the island regions spreads to the
surrounding region, and eventually the mineralizing
element-maldistributed layer is entirely crystallized. However,
when the vitreous silica of the island regions is different from
that of the surrounding region (i.e., when one is synthetic
vitreous silica, and the other is natural vitreous silica), the
crystallization started at the island regions hardly spread to the
surrounding region.
Furthermore, the non-doped inner surface layer does not
substantially contain a mineralizing element, and is made of
vitreous silica of a different kind from the vitreous silica of the
island region. Therefore, the crystallization started at the
mineralizing element-maldistributed layer does not spread to the
inner surface layer, and thus silicon melt is not contaminated.
In sum, according to the present invention, there is provided a
vitreous silica crucible which can suppress buckling and sidewall
lowering of the crucible and generation of cracks.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a sectional view showing a structure of vitreous silica
crucible of one embodiment of the present invention.
FIG. 2 is an enlarged view of region A in FIG. 1.
FIG. 3 is a sectional view for explaining the evaluation criteria
of the crucible in Examples.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
1. Configuration of Vitreous Silica Crucible
Hereinafter, with reference to FIGS. 1 and 2, embodiments of a
vitreous silica crucible of the present invention will be
explained. FIG. 1 is a sectional view showing a structure of a
vitreous silica crucible of the present embodiment, and FIG. 2 is
an enlarged view of the region A in the FIG. 1.
The vitreous silica crucible 1 of the present embodiment is a
vitreous silica crucible 1 for pulling a silicon single crystal,
and the wall 3 of the crucible 1 includes, from an inner surface
side toward an outer surface side of the crucible 1, a non-doped
inner surface layer 3a made of natural vitreous silica or synthetic
vitreous silica, a mineralizing element-maldistributed layer 3b
containing dispersed island regions each containing a mineralizing
element, and a mineralizing element uniformly-distributed vitreous
silica layer 3c containing a substantially uniformly dispersed
mineralizing element (hereinafter referred to as "mineralizing
element uniformly-distributed layer"). The vitreous silica of the
island regions and the vitreous silica of the surrounding region is
a combination of mineralizing element-doped natural vitreous silica
and non-doped synthetic vitreous silica, or a combination of
mineralizing element-doped synthetic vitreous silica and non-doped
natural vitreous silica, and the inner surface layer is made of
vitreous silica of a different kind from the vitreous silica of the
island region.
Here, the respective components are explained in detail.
(1) Vitreous Silica Crucible 1
The vitreous silica crucible 1 of the present embodiment is used
for pulling a silicon single crystal, and can be used for either
single pulling and multi-pulling, but it is preferred to be used
for multi-pulling. This is because the vitreous silica crucible 1
of the present embodiment solves problems which, as mentioned
above, are eminent in multi-pulling, more effectively than an
conventional crucible.
(2) Wall 3 of Vitreous Silica Crucible
As shown in the sectional view of FIG. 1, the wall 3 of vitreous
silica crucible 1 has a corner portion 32, a cylindrical sidewall
portion 31, and a bottom portion 33. The corner portion 32 has a
relatively large curvature. The sidewall portion 31 has a rim
portion having an upward opening. The bottom portion 33 is flat or
has a relatively small curvature, and is mortar-shaped. The wall 3
includes, from an inner surface side toward an outer surface side
of the crucible, a synthetic layer 3a, a mineralizing
element-maldistributed layer 3b, and a mineralizing element
uniformly-distributed layer 3c. In the present invention, the
corner portion refers to a portion connecting the sidewall portion
31 and the bottom portion 33, and starts at a point where a line
tangential to the corner portion 32 overlaps with the sidewall
portion 31 and ends at a point where the corner portion 32 and the
bottom portion 33 have a common tangential line. In other words,
the boundary between the sidewall portion 31 and the corner portion
32 is a point where a straight portion of the wall 3 starts to
curve. Furthermore, the portion with a constant curvature at the
bottom of the crucible is the bottom portion 33, and as the
distance from the center of the crucible increases, a point where
the curvature starts to change is the boundary between the bottom
portion 33 and the corner portion 32.
(2-1) Inner Surface Layer 3a
The non-doped inner surface layer 3a is an innermost layer of the
crucible 1, and contacts silicon melt. In the present
specification, "Non-doped" used herein means that the impurity
concentration is 20 ppm or less (preferably 15 ppm or less). The
inner surface layer 3a is made of natural vitreous silica or
synthetic vitreous silica. Natural vitreous silica is vitreous
silica obtained by fusing and solidifying silica powder obtained
from a natural mineral whose main component is .alpha.-quartz.
Synthetic vitreous silica is vitreous silica obtained by fusing and
solidifying chemically synthesized amorphous or crystalline silica
(silicon oxide) powder, and has very low impurity concentration.
Therefore, it is possible to reduce the amount of impurities mixed
in silicon melt by providing the inner surface layer 3a made of
synthetic vitreous silica. The method of chemical synthesis of
silica is not in particular limited, and, may be, for example, gas
phase oxidation (dry synthesis) of silicon tetrachloride
(SiCl.sub.4), or hydrolysis (sol-gel method) of silicon alkoxide
(Si(OR).sub.4). The inner surface layer 3a gradually corrodes
through the reaction with silicon melt during pulling of a silicon
ingot. Therefore, the inner surface layer 3a needs to have a
thickness sufficient to remain until the end of the pulling of a
silicon ingot, and the thickness is preferred to be, for example,
approximately 0.8 to 2 mm.
(2-2) Mineralizing Element-Maldistributed Layer 3b
The mineralizing element-maldistributed layer 3b is a layer
disposed on the outer side of the inner surface layer 3a, and is a
vitreous silica layer containing dispersed island regions each
containing a mineralizing element. The mineralizing element has a
function to promote crystallization of vitreous silica, and thus
vitreous silica is crystallized during pulling of a silicon ingot
at the island regions containing a mineralizing element.
The vitreous silica of the island regions and the vitreous silica
of the surrounding region is a combination of mineralizing
element-doped natural vitreous silica and non-doped synthetic
vitreous silica, or a combination of mineralizing element-doped
synthetic vitreous silica and non-doped natural vitreous silica.
When both of the vitreous silica of the island regions and the
vitreous silica of the surrounding region are natural vitreous
silica or both are synthetic vitreous silica, the crystallization
started at the island regions spreads to the surrounding region,
and eventually the mineralizing element-maldistributed layer is
entirely crystallized. However, when the vitreous silica of the
island regions is different from that of the surrounding region
(i.e., when one is synthetic vitreous silica, and the other is
natural vitreous silica), the crystallization started at the island
regions hardly spread to the surrounding region.
Furthermore, when the vitreous silica of the inner surface layer 3a
is the same kind as that of the island region, there occurs a
problem that the crystallization started at the island regions
spreads to the inner surface layer 3a, and thus the vitreous silica
of the inner surface layer 3a needs to be of a different kind from
that of the island region. Therefore, when the inner surface layer
3a is made of synthetic vitreous silica, the vitreous silica of the
island regions and the vitreous silica of the surrounding region
needs to be a combination of mineralizing element-doped natural
vitreous silica and non-doped synthetic vitreous silica, and when
the inner surface layer 3a is made of natural vitreous silica, the
vitreous silica of the island regions and the vitreous silica of
the surrounding region needs to be a combination of mineralizing
element-doped synthetic vitreous silica and non-doped natural
vitreous silica.
When the size of the island regions is not in particular limited,
but when the island regions is too small, the improvement of the
crucible strength becomes insufficient, and when the island regions
is too large, cracks are easier to be formed in the crucible. Thus,
the average diameter of the island regions is preferred to be 50
.mu.m to 1 mm, and more preferred to be 100 to 800 .mu.m, and even
more preferred to be 200 to 500 .mu.m. In general, the average
diameter of silica powder to form a vitreous silica layer is
approximately 200 to 400 .mu.m, and the spread of the mineralizing
element during fusing the silica powder is slight. Therefore, it is
easy to form island regions having an average diameter of 200 to
500 .mu.m by fusing mixed silica powder obtained by mixing
mineralizing element-doped silica powder and non-doped silica
powder to form the mineralizing element-maldistributed layer.
Furthermore, by use of silica powder obtained by doping, with a
mineralizing element, smaller or larger silica powder, it is
possible to form island regions having an average diameter of 50
.mu.m to 1 mm. The island regions may be formed by other
methods.
"Particle size" is, in general, as shown in the section of the term
definition of "Test Powder and Test Particles" in JIS Z 8901, a
size represented by the aperture size of a test sieve used for the
measurement in the screening method, a size represented by the
Stokes equivalent diameter obtained by the sedimentation method, a
size represented by a circle equivalent diameter obtained in the
microscope method, a size represented by a sphere equivalent
diameter obtained by the light scattering method, or a size
represented by a sphere equivalent diameter obtained by the
electrical resistance test, and is also referred to as "particle
diameter." However, in the present specification, the particle size
distribution is measured by use of the laser diffraction/scattering
measurement method using laser light as a light source.
The principle is to utilize a phenomenon that when particles are
irradiated with light, the intensity and pattern of the light
scattered by each particle changes depending on the particle
diameter (Mie scattering). When the particle diameter is large, the
intensity of the scattered light in all direction is strong, and
the intensity of the forward scattered light is in particular
strong. As the particle diameter decreases, the overall scattered
light intensity weakens, and the forward-scattered light is only
weakly detected. Therefore, when the particle diameter is large,
the forward-scattered light collected by a convex lens generates a
diffraction pattern on the focal plane. The brightness and size of
the diffracted light depends on the particle size (particle
diameter). Therefore, by use of information from the scattered
light, the particle diameter can be obtained easily.
In contrast, when the particle diameter decreases, the intensity of
the forward-scattered light weakens, and thus it is difficult to
detect the light by use of a detector mounted in front. However, as
the scattering pattern of the side-way and back scattered light
changes depending on the particle diameter, it is possible to
determine the particle diameter by measuring these. The measurement
result is compared with a spherical particle exhibiting a
scattering pattern equivalent to the scattering pattern for the
measured particle, and the result is outputted as a particle size
distribution. Therefore, for example, when a measured particle
exhibits a diffracted/scattered light pattern equivalent to a
sphere having a diameter of 1 .mu.m, the diameter of the particle
is determined to be 1 .mu.m irrespective of the shape. The diameter
is different from that determined by other measurement methods
using visual or image analysis, such as "Feret diameter"
corresponding to the length in a specific axis direction of
randomly oriented particles, "equivalent diameter" corresponding to
the size of a particle of an ideal shape (usually a circle) which
has the same area as the projected area of the particle, or an
aspect ratio representing the ratio of the long axis and short
axis. Furthermore, the "average particle diameter" represents a
particle diameter at an integrated value of 50% in the obtained
particle size distribution.
The outer edge of the island region can be specified by determining
the concentration maximum value of the mineralizing element in the
island region, connecting points having one tenth ( 1/10) of the
maximum value, and measuring the diameter of the circumscribed
circle surrounding the connected points. The diameter of the
circumscribed circle is the diameter of the island region. The
average diameter of the island regions is an average value of the
diameters of the neighboring ten island regions. The concentration
at a micro region can be measured by using Secondary Ion Mass
Spectrometry (SIMS) which can determine distribution and quantity
of respective elements by irradiating a sample with ions, and
analyzing, by mass spectrometry, secondary ions released from the
surface of the sample by sputtering.
Specifically, a sample having a square of 10 mm.times.10 mm and a
thickness of 3 mm is cut out from the crucible, and the sample is
set on a sample holder in a way that a surface, of the sample,
vertical to the inner surface of the crucible is irradiated with
primary ions. Then, the sample is irradiated with the primary ions
of oxygen (O.sup.2+) or cesium (Cs.sup.+) under vacuum atmosphere.
Then, secondary ions released by the irradiation of the primary
ions are analyzed by mass spectrometry to identify elements
constituting the sample. Then, the concentrations of the respective
elements of the sample can be quantitatively analyzed by the ratio
of the strength of the secondary ions of the sample and the
strength of the secondary ions released from a standard sample
(concentrations of constituent elements of the standard sample are
known).
The concentration maximum value of a mineralizing element in the
island regions is preferred to be 20 to 600 ppm. When the
concentration is too low, the crystallization becomes too slow, and
when the concentration is too high, the crystallization can spread
to the surrounding region.
The mineralizing element used herein refers to an element which
promotes crystallization of vitreous silica. It can exist in
vitreous silica in the form of an inorganic salt, nitrate,
carbonate, sulfate, acetate, oxalate, fluoride salt, phosphate,
oxide, peroxide, hydroxide, chloride, or may be substituted for Si
of vitreous silica, or in the ionized state, or in the state of a
compound such as Al.sub.2O.sub.3.2SiO.sub.2. The kind of the
mineralizing element is not in particular limited as long as the
mineralizing element promotes crystallization of the vitreous
silica. The kind of the mineralizing element is preferably metal
impurities, because metal impurities particularly promote
crystallization. The kind of the mineralizing element is, for
example, alkali metal (e.g., sodium or potassium), alkali earth
metal (magnesium, calcium, strontium, or barium), aluminium, or
iron. When aluminium is added, the viscosity of vitreous silica is
enhanced, and thus the mineralizing element is preferred to be
aluminium.
The mineralizing element-maldistributed layer 3b may be in contact
with the inner surface layer 3a, or there may be provided another
layer (e.g., natural vitreous silica layer (hereinafter referred to
as "natural layer") between the mineralizing element-maldistributed
layer 3b and the inner surface layer 3a. The natural layer is a
layer formed of vitreous silica obtained by fusing and solidifying
silica powder obtained from natural mineral whose main component is
.alpha.-quartz. When .alpha.-quartz is fused, the viscosity is
largely reduced. However, the chain structure of the repetition of
SiO bond is not completely destroyed, and thus natural vitreous
silica still contains crystalline microstructure therein, and thus
natural vitreous silica is not easily deformed. Thus, the natural
layer has relatively high viscosity.
The mineralizing element-maldistributed layer 3b in the sidewall
portion 31 needs to have a thickness enough to maintain the
crucible strength, and the thickness is, for example, approximately
4 to 15 mm. When the mineralizing element-maldistributed layer 3b
is too thin, the improvement of the crucible strength is
insufficient, and when it is too thick, the total wall thickness
becomes too large.
(2-3) Mineralizing Element Uniformly-Distributed Layer 3c
The mineralizing element uniformly-distributed layer 3c is a layer
optionally provided on the outer side of the mineralizing
element-maldistributed layer 3b, and contains a substantially
uniformly dispersed mineralizing element. The mineralizing element
uniformly-distributed layer 3c can be omitted. The mineralizing
element uniformly-distributed layer 3c is crystallized at an early
stage of the pulling of a silicon ingot, and enhances the crucible
strength. "Substantially uniformly" means that a mineralizing
element is uniformly dispersed to an extent that the mineralizing
element crystallizes the mineralizing element uniformly-distributed
layer in its entirety. Therefore, in the mineralizing element
uniformly-distributed layer 3c, although the mineralizing element
is preferred to be uniformly dispersed, but it does not have to be
completely uniformly dispersed, and there may be some nonuniformity
in the concentration.
The concentration of the mineralizing element in the mineralizing
element-maldistributed layer 3c is not in particular limited, but
is preferred to be 20 to 600 ppm. When the concentration is too
low, the crystallization becomes too slow, and when the
concentration is too high, the crystallization becomes too fast,
which may become a cause of formation of cracks or the mineralizing
element may be dispersed to reach silicon melt. Furthermore, the
mineralizing element uniformly-distributed layer 3c is preferred to
be crystallized at an earlier stage than the mineralizing
element-maldistributed layer 3b, and thus it is preferred that the
mineralizing element uniformly-distributed layer 3c contains a
mineralizing element in a higher concentration than the
concentration maximum value of the mineralizing element in the
island region. The mineralizing elements that can be included in
the mineralizing element-maldistributed layer 3c are the same as
those of the mineralizing element-maldistributed layer 3b. The
mineralizing element in the mineralizing element
uniformly-distributed layer 3c can be the same as or different from
that of mineralizing element-maldistributed layer 3b.
The vitreous silica of the mineralizing element
uniformly-distributed layer 3c is of a different kind from that of
the surrounding region of the island regions (one is natural
vitreous silica, and the other is synthetic vitreous silica). When
these vitreous silicas are the same, the crystallization started at
the mineralizing element uniformly-distributed layer 3c spreads to
the entire mineralizing element-maldistributed layer 3b, and thus
the mineralizing element-maldistributed layer 3b is entirely
crystallized, which leads to formation of cracks. Therefore, when
the vitreous silica of the surrounding region of the island regions
is synthetic vitreous silica, the vitreous silica of the
mineralizing element uniformly-distributed layer 3c needs to be
natural vitreous silica, and in contrast, when the vitreous silica
of the surrounding region of the island regions is natural vitreous
silica, the vitreous silica of the mineralizing element
uniformly-distributed layer 3c needs to be synthetic vitreous
silica. The mineralizing element uniformly-distributed layer 3c is
preferred to be natural vitreous silica in view of enhancing the
high-temperature strength of the crucible.
The thickness of the mineralizing element uniformly-distributed
layer 3c in the sidewall portion 31 is, for example, 1 to 4 mm.
When the mineralizing element uniformly-distributed layer 3c is too
thin, the improvement of the crucible strength is insufficient, and
when it is too thick, cracks become more likely to be formed in the
crucible.
2. Method of Manufacturing Vitreous Silica Crucible
The vitreous silica crucible 1 of the present embodiment can be
manufactured by the processes of (1) forming a silica powder layer
for a mineralizing element uniformly-distributed layer 3c, a
mineralizing element-maldistributed layer 3b, and an inner surface
layer 3a, by depositing crystalline or amorphous silica powder on
the inner surface (the bottom surface and the side surface) of a
rotating mold, and (2) vitrifying the silica powder layer by
heating and fusing the silica power layer up to a temperature of
2000 to 2600 degrees C. by use of arc discharge, followed by
cooling.
The silica powder to form the natural vitreous silica (natural
silica powder) can be manufactured by pulverizing natural mineral
whose main component is .alpha.-quartz.
The silica powder to form the synthetic vitreous silica (synthetic
silica powder) can be manufactured by chemical synthesis, such as
gas phase oxidation (dry synthesis) of silicon tetrachloride
(SiCl.sub.4), or hydrolysis (sol-gel method) of silicon alkoxide
(Si(OR).sub.4).
The mineralizing element-maldistributed layer 3b can be formed by
fusing mixed silica powder obtained by mixing mineralizing
element-doped natural silica powder and non-doped synthetic silica
powder, or mineralizing element-doped synthetic silica powder and
non-doped natural silica powder in a ratio of 1:1 to 1:100. When
the ratio of the mineralizing element-doped silica powder
(hereinafter referred to as "doped silica powder") is larger than
1:1, particles of the doped silica powder contact each other to
form a too large island-like crystal, which results in the
crystallization of the mineralizing element-maldistributed layer 3b
in its entirety. Furthermore, when the ratio of the non-doped
silica powder is larger than 1:100, the number of the island-like
crystals becomes too small, and thus the improvement of the
crucible strength can become insufficient. Therefore, the ratio is
1:1 to 1:100. The ratio of the doped silica powder and non-doped
silica powder is preferred to be 1:3 to 1:20. The ratio is for
example, 1:1, 1:3, 1:5, 1:10, 1:15, 1:20, 1:30, 1:50, or 1:100, and
it can be in the range between two values of the values exemplified
here. The doped silica powder can be formed by, for example, firing
the mixture of silica powder and metal alkoxide under nitrogen
atmosphere at 600 to 1100 degrees C.
A transparent layer having virtually no bubbles (i.e., bubble
content of less than 0.5%) can be formed by subjecting the silica
powder layer to a reduced pressure of -50 kPa or more and less than
-95 kPa while fusing the silica powder layer. Furthermore, after
the transparent layer is formed, a bubble-containing layer having a
bubble content of 0.5% or more and less than 50% can be formed on
the outer side of the transparent layer by subjecting the silica
powder layer to a pressure of +10 kPa or more and less than -20
kPa. In the present specification, the bubble content rate refers
to the ratio (w.sub.2/w.sub.1) of the volume (w.sub.2) occupied by
bubbles in a unit volume (w.sub.1) of the crucible 1. In the
present specification, the value of the pressure is the value with
reference to the ambient air pressure.
3. Method of Manufacturing Silicon Ingot
A silicon ingot can be manufactured by the processes of (1) forming
silicon melt by melting polycrystalline silicon in the vitreous
silica crucible 1 of the present embodiment, and (2) dipping an end
of a silicon seed crystal to the silicon melt, and pulling the seed
crystal while rotating the seed crystal. The silicon single crystal
has a shape having, from the upper side, a cylindrical silicon seed
crystal, a cone-shaped silicon single crystal, a cylindrical
silicon single crystal having the same diameter as the base of the
upper cone (hereinafter, referred to as "straight body portion"), a
cone-shaped silicon single crystal having a downward apex.
When multi-pulling is performed, polycrystalline silicon is
recharged and melted in the vitreous silica crucible 1, and
thereafter another silicon ingot is pulled.
The pulling of a silicon ingot is carried out usually at
approximately 1450 to 1500 degrees C. When a crucible is exposed to
such high temperature for a long time, the vitreous silica of the
crucible is crystallized at a portion containing a mineralizing
element. When the mineralizing element uniformly-distributed layer
3c is provided on the outer surface side, the mineralizing element
uniformly-distributed layer 3c is first crystallized, and
thereafter island-like crystallization occurs in the mineralizing
element-maldistributed layer 3b. When the mineralizing element
uniformly-distributed layer 3c is not provided, crystallization
starts from the island-like crystallization in the mineralizing
element-maldistributed layer 3b. Because vitreous silica, which is
relatively easily deformed, exists between the island-like
crystals, the island-like crystals enhance the crucible strength
while preventing formation of cracks in the crucible.
EXAMPLE
According to the following procedures, there are evaluated the
crystallinity of a silicon ingot, formation of cracks in the
crucible, and the crucible strength.
A crucible having an outer diameter of 800 mm and a wall thickness
of 15 mm was manufactured. The crucibles of Examples and
Comparative Examples were manufactured so as to have vitreous
silica layers shown in Table 1. The order of the respective
vitreous silica layers from the left to the right in Table 1
denotes the order from the inner surface to the outer surface of
the crucible. The mineralizing element uniformly-distributed layer
was formed by use of doped silica powder obtained by adding Al as
impurities to natural silica powder. The Al concentration was
adjusted to achieve the mineralizing element concentration shown in
Table 1.
Furthermore, the mineralizing element-maldistributed layer was
formed by use of mixed silica powder obtained by mixing doped
silica powder and non-doped silica powder in the ratio shown in
Table 1. In Table 1, "D Natural" and "D Synthetic" denote
mineralizing element-doped natural silica powder and synthetic
silica powder, respectively. "ND Natural" and "ND Synthetic" denote
non-doped natural silica powder and synthetic silica powder,
respectively. The value of the ratio "Synthetic Natural" denotes
the mixing ratio of the silica powders. For example, "D Synthetic:
ND Natural=1:5" means that doped synthetic silica powder and
non-doped natural silica powder were mixed in a ratio of 1:5. The
average diameter of the non-doped silica powder was 400 .mu.m. The
size of the doped silica powder was adjusted so as to obtain the
average diameter of the island regions shown in Table 1.
TABLE-US-00001 TABLE 1 Mineralizing Element Uniformly-Distributed
Layer Inner Mineralizing Element-Maldistributed Layer (3b) (3c) ND
Surface Concen- Average Concen- Natural Layer (3a) Thickness D
Silica Powder: tration Diameter Thickness tration Layer (mm) (mm)
ND Silica Powder (ppm) (.mu.m) (mm) (ppm) (mm) Ex. 1 Synthetic 14 D
Natural: 30 200 0 -- 0 Layer ND Synthetic = 1 mm 1:2 Ex. 2
Synthetic 11 D Natural: 30 200 Natural Layer 50 0 Layer ND
Synthetic = 3 mm 1 mm 1:2 Ex. 3 Synthetic 11 D Natural: 30 200
Natural Layer 50 0 Layer ND Synthetic = 3 mm 1 mm 1:90 Ex. 4
Synthetic 11 D Natural: 450 200 Natural Layer 500 0 Layer ND
Synthetic = 3 mm 1 mm 1:2 Ex. 5 Synthetic 11 D Natural: 30 900
Natural Layer 50 0 Layer ND Synthetic = 3 mm 1 mm 1:2 Ex. 6
Synthetic 11 D Natural: 30 200 Natural Layer 50 0 Layer ND
Synthetic = 3 mm 1 mm 1:200 Ex. 7 Synthetic 11 D Natural: 10 200
Natural Layer 12 0 Layer ND Synthetic = 3 mm 1 mm 1:2 Ex. 8
Synthetic 11 D Natural: 30 1 Natural Layer 50 0 Layer ND Synthetic
= 3 mm 1 mm 1:2 Ex. 9 Natural 11 D Synthetic: 30 200 Synthetic 50 0
Layer ND Natural = Layer 1 mm 1:2 3 mm Ex. 10 Natural 11 D
Synthetic: 30 200 Synthetic 50 0 Layer ND Natural = Layer 1 mm 1:90
3 mm Ex. 11 Natural 11 D Synthetic: 30 200 Synthetic 50 0 Layer ND
Natural = Layer 1 mm 1:200 3 mm Comp. Synthetic 0 -- -- -- -- 14
Ex. 1 Layer 1 mm Comp. Synthetic 11 D Natural: 30 200 Natural Layer
50 0 Ex. 2 Layer ND Synthetic = 3 mm 1 mm 1:0.5 Comp. Synthetic 11
D Natural: 600 200 Natural Layer 700 0 Ex. 3 Layer ND Synthetic = 3
mm 1 mm 1:2 Comp. Synthetic 11 D Natural: 30 1200 Natural Layer 50
0 Ex. 4 Layer ND Synthetic = 3 mm 1 mm 1:2 Comp. Natural 11 D
Synthetic: 30 200 Synthetic 50 0 Ex. 5 Layer ND Natural = Layer 1
mm 1:0.5 3 mm
Three silicon ingots each having a diameter of 300 mm were pulled
by use of the crucibles of Examples and Comparative Examples. Each
time one silicon ingot was pulled, polycrystalline silicon was
recharged and melted. Crystallinity of the three silicon ingots was
evaluated. Evaluation of the crystallinity was performed based on
the single crystallization yield. The single crystallization yield
was a value of (mass of the straight body section of the silicon
single crystal)/(mass of silicon melt charged in the crucible right
before pulling). The results are shown in Table 2. The evaluation
criteria in Table 2 are as follows: A: single crystallization yield
is 0.80 or more and less than 0.99 B: single crystallization yield
is 0.70 or more and less than 0.80 C: single crystallization yield
is 0.60 or more and less than 0.70 D: single crystallization yield
is less than 0.60
The crucible strength was evaluated based on the amount of sidewall
lowering obtained by measuring, before and after use, the distance
from the reference level (which is the upper end of the carbon
susceptor 5) to the upper end of the crucible 1 (See FIG. 3). The
results are shown in Table 2. The evaluation criteria are shown
below.
Evaluation Criteria
A: the amount of sidewall lowering is less than 10 mm B: the amount
of sidewall lowering is 10 mm or more and less than 20 mm C: the
amount of sidewall lowering is 20 mm or more and less than 30 mm D:
the amount of sidewall lowering is 30 mm or more
TABLE-US-00002 TABLE 2 Crystalliza- tion of Mineralizing Element-
Single Crystallization Yield Maldistrib- First Second Third uted
Layer Cracks? Strength Ingot Ingot Ingot Ex. 1 Island Not C A B B
Formed Ex. 2 Island Not A A A A Formed Ex. 3 Island Not B A A B
Formed Ex. 4 Island Not A A A A Formed Ex. 5 Island Not A A A A
Formed Ex. 6 Island Not D B C D Formed Ex. 7 Island Not D B C D
Formed Ex. 8 Island Not D B C D Formed Ex. 9 Island Not A B B B
Formed Ex. 10 Island Not A B B B Formed Ex. 11 Island Not A B B B
Formed Comp. -- Not D B D Not Ex. 1 Formed Obtained Comp. Entire
Formed A A Not -- Ex. 2 Obtained Comp. Entire Formed A A Not -- Ex.
3 Obtained Comp. Entire Formed A A Not -- Ex. 4 Obtained Comp.
Entire Formed B B Not -- Ex. 5 Obtained
After pulling of three silicon ingots, the crucible was inspected
to find that the island-like crystals were formed in the
mineralizing element-maldistributed layer of the crucible of
Examples.
When the crucible of Example 1 was used, the single crystallization
yield of the first ingot was high, and the single crystallization
yield of the second and third ingots slightly deteriorated. No
cracks were observed, and a sidewall lowering of 25 mm was
observed.
When the crucible of Examples 2, 4, or 5 was used, the single
crystallization yield of the first, second, and third ingots were
high. No cracks were observed, and a sidewall lowering of less than
10 mm was observed.
When the crucible of Example 3 was used, the single crystallization
yield of the first and second ingots were high, and the single
crystallization yield of the third ingot slightly deteriorated. No
cracks were observed, and a sidewall lowering of 15 mm was
observed.
Example 6 is different from Example 2 in that the ratio of the
doped silica powder was decreased. In this Example, the ratio of
the doped silica powder was too low, and thus even though
island-like crystals were formed in the mineralizing
element-maldistributed layer, the number was not enough to
sufficiently enhance the crucible strength.
Example 7 is different from Example 2 in that the mineralizing
element concentration was lowered. Crystallization of the crucible
of Example 6 was insufficient, and thus the crucible strength was
not sufficiently enhanced.
Example 8 is different from Example 2 in that the size of the
island regions was decreased. In the crucible of Example 8,
although the island-like crystals were formed, the size was small,
and thus the crucible strength was not sufficiently enhanced.
Examples 9 to 11 are different from Examples 2, 3, and 6,
respectively, in that natural silica powder was replaced with
synthetic silica powder, and synthetic silica powder was replaced
with natural silica powder. In the crucible of Examples 9 to 11,
island-like crystals were formed in the mineralizing
element-maldistributed layer, and formation of cracks was
suppressed.
When the crucible of Comparative Example 1 was used, the single
crystallization yield of the first ingot slightly deteriorated, and
single crystal was hardly obtained in the second ingot. During
pulling of the third silicon ingot, the non-doped natural layer was
not crystallized, and thus sidewall lowering was large, and then
the pulling of a single crystal was stopped. No cracks were
observed.
In Comparative Example 2, the ratio of doped silica powder was too
high, and thus the mineralizing element was not maldistributed in
island-like form in the mineralizing element-maldistributed layer,
and thus the layer was crystallized in its entirety, and thus
cracks were formed.
In Comparative Example 3, the mineralizing element concentration in
the doped silica powder was too high, and thus the mineralizing
element was not maldistributed in island-like form in the
mineralizing element-maldistributed layer, and the layer was
crystallized in its entirety, and thus cracks were formed.
In Comparative Example 4, the size of the doped silica powder was
too large, and thus the mineralizing element was not maldistributed
in island-like form in the mineralizing element-maldistributed
layer, and the layer was crystallized in its entirety, and thus
cracks were formed.
Comparative Example 5 is different from Comparative Example 2 in
that natural silica powder was replaced with synthetic silica
powder, and synthetic silica powder was replaced with natural
silica powder. In the crucible of Comparative Example 5 as well as
Comparative Example 2, the mineralizing element-maldistributed
layer was crystallized in its entirety.
* * * * *